The invention pertains to apparatus for dividing a gas stream into several partial gas streams, the apparatus having an equalization chamber with a gas inlet for the gas stream and several gas outlet openings for the partial gas streams. Each of these outlet openings is connected by way of a gas line to a gas consumer, the maximum flow resistance for each partial gas stream being provided in the area which, looking in the flow direction of the partial gas streams, is under the gas outlets.
Apparatus such as this, referred to in the following as a "flow divider", is known from U.S. Pat. No. 5,116,400. The flow divider described therein is used to supply a multi-burner assembly with burner gases. The multi-burner assembly is used to deposit SiO.sub.2 soot onto an elongated support body, which is rotated around its longitudinal axis. For this purpose, the burners are guided axially back and forth for a short distance along the support body, this distance being approximately equal to that between the individual burners.
To produce a homogeneous soot body, it is important for each burner to build up the deposit at the same rate as all the other burners. If the individual burners have different build-up rates, the external geometry of the soot body will be irregular. Unequal build-up rates of the individual burners can be caused by fluctuations in the gas supply or by production-related deviations from ideal burner geometry. To guarantee the same build-up rate for all burners of a multi-burner assembly, U.S. Pat. No. 5,116,400 proposes that an equalization tank be provided in the gas supply system, the tank being provided with a gas inlet and with a number of gas outlets corresponding to the number of burners. The opening of the gas inlet has a larger cross section than that of the various gas outlets. The area of the maximum pressure drop should accordingly be located in the part of the burner where the burners are as similar as possible to each other. This part then corresponds at the same time to the maximum flow resistance of the gas supply system.
In the production of quartz glass by means of the soot deposition process, corrosive media such as silicon tetrachloride (SiCl.sub.4) are often required. Burners made of metal are chemically attacked by these media, or they are warped by the high temperatures. The burners are therefore normally made of quartz glass. In the case of quartz glass tubes, however, it is difficult to arrive at precisely the required inside diameter and therefore difficult to arrive at precisely the desired maximum flow resistance.
But even under the assumption that the burners all have the same geometry, the known device does not guarantee that all the burners will have approximately the same build-up rate relative to each other at every moment of the soot deposition process. It has been found that pressure differences develop inside the equalization tank, with the result that different pressures are reached at the gas outlets. This necessarily leads in the case of geometrically identical burners to different build-up rates relative to each other.
It is also possible, however, for the absolute build-up rate of all the burners to vary over the course of soot body buildup as a result of variations in the gas volume supplied through the gas inlet. The conventional method of controlling the burner gas feed is to use rotameters or mass flow controllers (MFC's). The principle of mass flow control is based on the conduction of heat through the gas flow to be measured. Temperature changes in the area of the MFC's or changes in the MFC as a result of corrosion by aggressive media such as SiCl.sub.4, for example, can cause a drift in the control function over time. A drift such as this in the controlled flow rate of the gas being supplied by way of the MFC's brings about a change in the deposition rate of all the burners and causes an axial inhomogeneity in the microstructure of the soot body.
The invention is therefore based on task of providing a device which can be manufactured easily and at low cost and which guarantees the highest possible degree of uniformity and reproducibility of the gas feed to the gas consumers.